This invention relates generally to an improved film transport assembly for intermittently advancing film one frame at a time to a film gate in a motion picture camera or projector or the like. More particularly, this invention provides a film transport assembly for high speed intermittent film advancement and/or advancement of relatively large film formats while providing an extended cycle dwell or exposure period for each film frame at a film gate.
Motion picture cameras and projectors and the like are well known wherein an elongated strip of film is advanced from a supply reel for passage through a film gate and subsequent winding onto a takeup reel. The supply and takeup reels are normally driven by sprocket drives or the like to advance the film at a substantially constant or linear velocity respectively to and from the film gate. However, between these reels and the film gate, the film is typically displaced through relatively free loops which permit a conventional pulldown claw mechanism or the like adjacent the film gate to displace the film past the gate with intermittent motion. In this manner, each film frame is halted at the film gate for a brief dwell period to permit film exposure or projecting of an image therefrom before film transport to the next film frame in sequence. While this free loop film transport arrangement is widely used and functions adequately for many standard film format sizes and film transport speeds, such free loop arrangements do not function well with large film formats requiring rapid transport speeds between frames or with cameras adapted for high speed operation, for example, on the order of several hundred frames per second or more. In particular, at these operating conditions, the use of free film loops tends to result in undesirable instabilities during transport including loop flutter and/or vibrations, resulting in imprecise frame-to-frame registration at the film gate.
Various mechanisms have been proposed for permitting increased film transport or increased cycle speeds without introducing mechanical instabilities. Such mechanisms have included, for example, a variety of resilient means such as spring devices and the like for holding the otherwise free film loops under tension during film transport. However, such approaches are susceptible to uncontrolled vibration and flutter due to natural mechanical resonant frequencies. In addition, such use of resilient or spring devices undesirably applies excessive forces to the film resulting in excessive film wear and frequent incidents of film breakage.
One improved intermittent film transport mechanism is shown and described in U.S. Pat. No. 3,613,978. In particular, this patent discloses a captive loop transport arrangement wherein film is advanced at constant speed respectively to and from a film gate by conventional supply and takeup sprockets. Between each sprocket and the gate, however, the film is passed in a captive manner over two rollers which are displaced according to sinusoidal displacement functions and at the same cyclic rates. These rollers are designed to increase the film path length between the supply sprocket and gate while simultaneously decreasing the gate-takeup sprocket path length in a manner stopping the film at the gate for a dwell period of 90 degrees of the total frame-to-frame cycle, followed by film transport over the remainder of said cycle to register the next film frame with the gate. However, while this film transport arrangement functions successfully for its intended purpose, the dwell period of up to 90 degrees can be too short particularly wherein relatively high transport speeds are desired, for example, speeds on the order of 500 to 1,000 frames per second or more.
The present invention also utilizes two moving rollers on the supply and takeup sides of a film gate, although one variation of the invention may use the dual rollers on the takeup side only. However, the invention improves upon prior apparatus by moving the roller sets at different cyclic rates and amplitudes to achieve an extended dwell period of substantially one half, or 180 degrees, of the total frame-to-frame cycle.